The present invention relates to an optical pick-up apparatus and in particular, to an optical pick-up apparatus for providing a preferable reproducing signal.
In general, in an optical pick-up apparatus, an optical path from a semiconductor laser to a half mirror is inclined at a 45.degree. angle with respect to a track direction of a disk, as is disclosed in Japanese Patent Application Laid-Open No. 61-39942 for example.
In this optical pick-up apparatus, as shown in FIGS. 1 and 2, light beam 2 emitted from semiconductor laser 1 is reflected by flat parallel half mirror 3 as an optical element and is converged on track 6 of disk 5 through objective lens 4. A portion of the light beam reflected by track 6 transmits half mirror 3 and astigmatism is thereby caused. Thus, elliptical light spots 8 and 8' deformed in accordance with the distance from objective lens 4 to disk 5 are formed on light receiving element 7 divided into four sections. Light spots 8 and 8' are utilized as a focus error signal so as to focus objective lens 4 on track 6 at any time.
At this time, as shown in FIG. 2, light receiving element 7 is disposed such that divisional lines 9 and 10 of light receiving element 7 are parallel and perpendicular to track direction 11 of disk 5 in order to perform the tracking control and remove the adverse effects on the focus error signal by the tracking control. Further, light receiving element 7 is formed such that the respective major axis directions of light spots 8 and 8' formed on light receiving element 7 are inclined at a 45.degree. angle with respect to divisional lines 9 and 10, i.e., with respect to track direction 11. Direction 13 of light beam 2 from semiconductor laser 1 to half mirror 3 is set to be inclined at a 45.degree. angle with respect to track direction 11.
In the above-mentioned optical pick-up apparatus, half mirror 3 is supported and fixed by adhesion, etc. to the interior of the apparatus, and the adhesive portion is shown by both end edge portions 14 and 15 of half mirror 3 as illustrated in FIG. 2. Therefore, there is a case in which an excessively fixing force is applied to half mirror 3 by the adhesion of this adhesive portion. In this case, as shown in FIG. 3, half mirror 3 is warped and deformed with center line 16 of the half mirror as a projected or recessed portion.
Since the above both end edge portions 14 and 15 are inclined at a 45.degree. angle with respect to track direction 11, center line 16 is inclined at a 45.degree. angle with respect to track direction. Accordingly, deforming direction 17 of the half mirror perpendicular to this center line is inclined at a 45.degree. angle with respect to track direction 11. Therefore, there is a case in which astigmatism inclined at a 45.degree. angle with respect to the track direction is caused in light beam 2 reflected by half mirror 3 due to distortion of a reflecting face of the half mirror by the deformation thereof. The astigmatism of reflecting light beam 2 sometimes causes astigmatism inclined at a 45.degree. angle with respect to track direction 11 in the light spots irradiated onto disk 5.
However, the above-mentioned astigmatism of the light spots irradiated on the disk sometimes has an effect on a reproducing signal of the track. In particular, when the astigmatic direction is inclined at a 45.degree. angle with respect to the track direction, a longitudinal axial direction of the elliptical light spot 8" would extend to cover the adjacent tracks as shown in FIG. 3A. As a result, as shown in the following table, the amount of cross talk from an adjacent track becomes largest, which is not preferable as a reproducing signal.
______________________________________ Angle between astigmatism and track Cross talk amount ______________________________________ 0.degree. 0.001 45.degree. 0.012 90.degree. 0.0003 ______________________________________
In a conventional method for supporting the half mirror in the optical pick-up apparatus, as mentioned above, the astigmatic direction of the light spots on the disk caused by the deformation of the half mirror is inclined at a 45.degree. angle with respect to the track direction, thereby generating a lot of cross talk.